There has been known, as a filter used for communication devices such as mobile phones, a ladder-type filter in which piezoelectric thin film resonators including a piezoelectric substance sandwiched between upper and lower electrodes are connected in a ladder format. A duplexer and a module including two or more filters may be installed in a communication device.
When large electrical power is input to the piezoelectric thin film resonator, non-linearity depending on the c-axis orientation of the piezoelectric substance causes secondary distortion (second harmonic) in output signals. As a method of reducing the secondary distortion, it has been known to serially divide a piezoelectric thin film resonator included in a ladder-type filter and make the electrodes of the divided resonators in the c-axis orientation direction or the polarization axis direction of the piezoelectric substance of the divided resonators have an identical electric potential, or to divide a piezoelectric thin film resonator in parallel and make the electrodes of the divided resonators in the c-axis orientation direction or the polarization axis direction of the piezoelectric substance of the divided resonators have opposite electric potentials as disclosed in, for example, Japanese Patent Application Publication Nos. 2008-85989 and 2009-10932.
When a piezoelectric thin film resonator is serially divided, resonators having an electrostatic capacitance of 2C are to be connected in series where C represents the electrostatic capacitance of the piezoelectric thin film resonator before division. This results in increase in the area of a resonance region. On the other hand, when the piezoelectric thin film resonator is divided in parallel, the increase in the area of the resonance region is inhibited. However, a wiring line that switches upper and lower electrodes sandwiching a piezoelectric substance is to be formed to make the electrodes in the c-axis orientation direction or the polarization axis direction of the piezoelectric substance of the divided resonators obtained by dividing a piezoelectric resonator included in a ladder-type filter in parallel have opposite electric potentials. Accordingly, the region in which the wiring line is to be formed needs securing, and thus the filter increases in size.